I have a custom field which has a lookup to contact. How do i extract the Id of the lookup and pass it on in a flow using a process builder ?
You can create a variable in your flow as input only. Lets say
AccountId. In your process builder when you select action as
Execute Flow and choose the invokable flow with no screens elements, this variable will be available in your process builder to assign and you can assign this value using a reference value type.